doc/go_spec: fix typo

Fixes #9445

Change-Id: If7abd4d4d41cdfd5cf677f03533c930c8b965a01
Reviewed-on: https://go-review.googlesource.com/2128Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of December 22, 2014",
+	"Subtitle": "Version of December 26, 2014",
 	"Path": "/ref/spec"
 }-->
 
@@ -5455,7 +5455,7 @@ const (
 	c2 = len([10]float64{2})         // [10]float64{2} contains no function calls
 	c3 = len([10]float64{c1})        // [10]float64{c1} contains no function calls
 	c4 = len([10]float64{imag(2i)})  // imag(2i) is a constant and no function call is issued
-	c5 = len([10]float64{imag(z)})   // invalid: imag(x) is a (non-constant) function call
+	c5 = len([10]float64{imag(z)})   // invalid: imag(z) is a (non-constant) function call
 )
 var z complex128
 </pre>